1. Field of the Invention
The present invention relates to cardiac assist devices, and more specifically, to intra-aortic balloon pumps and a method and apparatus for reducing the deflation time of the intra-aortic balloon.
2. The Prior Art
Intra-aortic balloon counterpulsation, or pumping (IAEP), is the most widely used type of cardiac assist, with more than 150,000 procedures per year world-wide. It is used to augment the pumping action of the heart in conjunction with open heart surgery or severe heart failure. An IABP consists generally of a catheter with a long narrow balloon at one end and a volume displacement pump at the other. The balloon is inserted into the femoral artery in the leg, and up the arterial tree into the descending aorta, the large artery coming directly from the heart. The volume displacement pump inflates and deflates the balloon once every heart beat at exact points in the cardiac cycle by shuttling a gas into and out of the balloon. The balloon is inflated just after the aortic valve closes and deflates just prior to the next beat. The IABP is generally in place for two to three days. However, in cases of severe heart failure and with no alternate treatment available, a few patients have remained on the device for long periods, up to several months.
The IABP augments the failing heart by two mechanisms. First, it increases blood flow to the coronary arteries which feed the heart itself. The balloon inflates just as the heart completes the blood ejection portion of its cycle and the aortic valve closes. The increased pressure caused by the balloon displacement pushes blood into the coronary arteries. Second, blood displaced by the inflating balloon augments the pressure from the heart to push blood forward out of the aorta. Just before the next blood ejection portion of the cycle, the balloon is deflated, reducing aortic blood pressure so that blood refills the volume of the aorta taken up by the previously inflated balloon at lower pressure, reducing the work load of the heart.
An IABP of the prior art includes a control system and a pneumatic system. The control system causes the balloon to inflate and deflate synchronously with the heart, while monitoring and displaying various parameters concerning the procedure. The control system has seven major functions. 1) It acquires, amplifies, and displays the patient's electrocardiogram (ECG), arterial pressure (AP), and heart rate measurements. (2) It acquires a synchronizing signal from the heart. Usually the ECG R-wave is used, but sometimes the upstroke of arterial pressure is used. (3) It monitors the acquired signals and alerts the operator if the signals are noisy or lost. (4) It provides a means for the operator to set inflation and deflation timing with reference to the AP waveform displayed on the pump monitor. (5) It inflates and deflates the balloon according to the operator settings and meters the correct volume of gas into the balloon so that it fully inflates but does not overinflate. (6) It monitors and displays the balloon gas pressure, analyzes the pressure waveform for unsafe conditions, e.g., gas leakage, kinked line, disconnected balloon, etc., and alerts the operator to these conditions. (7) It analyzes and compensates for irregular cardiac rhythms by adjusting the inflate/deflate timing appropriately.
The pneumatic system 8, shown in FIG. 1, consists of a drive 10, volume chamber 12, connecting tubing 14, catheter 16, balloon 18, shuttle gas supply 20, and pressure transducer 22. The drive 10 supplies force to inflate and deflate the balloon by moving a diaphragm 24 in the volume chamber 12 forward during inflation and backward during deflation. The drive 10 may consist of a compressor 26 and a vacuum pump 34 which produces drive pressure and vacuum for moving the diaphragm 24. In this configuration, reservoir tanks 28, 36 and a pressure regulation means 30, 38 are used to control the pressure and vacuum. Valves 32, 40 are used to apply the pressure and vacuum respectively during appropriate parts of the cardiac cycle. Sometimes a third valve 42 is used to vent the pressure prior to applying vacuum. Another type of drive uses a motor to produce a linear displacement of the volume chamber diaphragm to actuate inflation and deflation.
The volume chamber 12 controls the pumped volume of the balloon 18. It includes the diaphragm 24 that separates the drive from the shuttle gas and that moves the shuttle gas into and out of the balloon 18. The diaphragm 24 may be a flexible polymer elastomer driven by gas/vacuum, a rigid plate or piston driven by a motor, or other configuration. The connecting tubing 14 is a series of tubes that interconnect the volume chamber 12, shuttle gas supply 20, pressure transducer 22, and catheter 16. The catheter 16 is small bore tube that connects the balloon 18 to the remainder of the system 8 and runs primarily within the body. The pressure transducer 22 for monitoring gas pressure taps off of the connecting tubing 14 adjacent to the volume chamber 12. The typical balloon 18 is generally cylindrical with a long, narrow shape. The expanding outer surface does not stretch, but maintains a constant volume once inflated.
The shuttle gas supply 20 provides the gas that inflates the balloon. It includes a gas source 46 (usually a portable tank), a reducing pressure regulator means 48 to reduce the source pressure to a safe level, and valves 50, 52 for filling and venting gas from the system at the connecting tubing 14. Generally, the shuttle gas supply 20 also includes a tank pressure gauge and a blow-off valve to vent gas if the pressure exceeds a safe level.
Helium is the gas of choice. The catheter 16 is necessarily narrow so that it does not significantly interfere with blood flow in the arterial tree between the femoral artery and the aorta. Consequently, the catheter bore is very small. Helium, the smallest molecular weight, non-flammable gas, flows the quickest through the small bore, optimizing the inflation and deflation rate of the balloon 18.
The IABP is a closed system in which a fixed volume of gas is shuttled between the volume chamber 12 and the balloon 18 through the connecting tubing 14 and catheter 16. The diaphragm 24, powered by the drive 10, moves rapidly forward to provide the forward force to push the gas from the volume chamber 12. At deflation the operation is reversed: the diaphragm 24 moves rapidly backward, creating a partial vacuum in the volume chamber 12, pulling gas from the balloon 18 into the volume chamber 12. In this process, the rate of inflation or deflation is primarily determined by the pressure difference across the tubing 14 and catheter 16, the difference between the pressure in the balloon 18 and the pressure in the volume chamber 12. During deflation, the pressure in the balloon 18 is the AP, the pressure put on the balloon 18 by the blood in the aorta. During deflation, the pressure in the volume chamber 12 starts as a partial vacuum and rises to the pressure to which the volume chamber 12 was initially filled as gas flows from the balloon 18 to the volume chamber 12 as the balloon 18 deflates.
One measure of IABP performance is the rate at which the balloon 18 inflates and deflates. This performance is important when the heart rate is high or the cardiac rhythm is irregular. The deflate rate is particularly important in the case of an early beat in an arrhythmia situation. If the balloon 18 does not deflate before the heart begins its ejection cycle, the aortic blockage caused by the inflated balloon 18 puts an additional strain on what is already a weakened heart.
One prior art strategy for improving deflation time is to initially fill the gas system to a pressure lower than the ambient (atmospheric) pressure, so that there is a partial vacuum at full deflation and a strong pressure gradient remains across the catheter. Maintaining a partial vacuum throughout the entire deflated period, typically a substantial fraction of the heart interval, especially at high heart rates, has several drawbacks. If there should be a leak in the balloon, blood can be pulled into the system. If there should be a leak in the internal connections or connecting tubing, air can be pulled into the system. Also, leaks are more difficult to detect. The presence of a leak or of overfilling is determined by monitoring the gas pressure after complete deflation. When the deflated pressure is below ambient and the inflated pressure is above ambient, a leak will cause a gas gain at deflation and a gas loss at inflation, in effect, partially canceling each other out and making a leak much more difficult to detect. If the pressures during both inflation and deflation are above ambient, a leak will cause a loss when averaged over the entire cycle.
Thus, it is preferable to operate the system with a steady-state deflation pressure at or just above ambient. It is easier to do the initial fill and it is possible to operate in an emergency when no gas is available by breaking the closed system briefly and initializing from atmospheric pressure directly.